The present invention relates to the irradiation of food products using ionizing radiation such as x-rays. The invention is also applicable to irradiation in industrial, medical, sterilization or other fields in which efficient irradiation of materials is desired.
Diseases caused by contaminated food are one of the most widespread health problems in both the developed and developing countries throughout the world. The majority of these diseases, caused by biological agents such as bacteria, parasites and viral agents, are manifest by symptoms such as diarrhea, abdominal pain, nausea and vomiting. The Center for Disease Control estimates that foodborne diseases cause approximately 76 million illnesses, 325,000 hospitalizations, and 5,000 deaths annually in the United States (Food-Related Illness and Death in the United States, Center for Disease Control and Prevention, 1999.) More alarming is the fact that many of the pathogens of real concern today were not even recognized as causes of foodborne illness just twenty years ago.
These and other issues have been addressed by irradiation. The amount or dose of radiation to which food is exposed is a function of the duration of exposure to radiation, the density of the food, and the energy emitted by the irradiator. Relatively low doses can control trichnia in pork, inhibit ripening and extend shelf life of fruits and vegetables, and control insects and other pests. Higher doses control bacteria in poultry and other foods. Still higher doses control microorganisms in herbs, spices, teas, and other dried vegetables.
The following is exemplary of pathogen sensitivity in meat:
D10 ValueLog Reduction6D10 ReductionPathogen(kGy)at 4.5 kGyDose (kGy)Camphylocbacter0.18 251.1Clostridium0.5867.73.5E. coli O157:H70.25-0.4510-181.5-2.7Listeria0.40-0.64  7-11.22.4-3.8Salmonella0.48-0.706.4-9.42.9-4.2Staphylococcus0.45 102.7Toxoplama gondii0.40-0.706.4-11 2.4-4.2Trichinella spiralis0.30-0.607.5-15 1.8-3.6
D10 values represent the x-ray dose that will result in a ten-fold reduction in pathogen concentration. Considering E. coli bacterium, the above chart shows that an irradiation dose of 4.5 kGy would ensure at least a ten billion-fold or 10 log reduction (log reduction column in the foregoing table) in concentration.
Over forty governments have established regulations permitting the irradiation of a wide variety of foods. Within the United States, the Food and Drug Administration (FDA) has approved the use of ionizing radiation for pathogen reduction, food preservation and disinfection for approximately 80% of the food supply. Applications now under consideration for ready-to-eat foods and shellfish will, if approved as expected, allow for irradiation of virtually all foods excepting fin fish. Regulations governing the irradiation of food are published at 9 CFR 179.26. Exemplary doses for certain foods are summarized below:
Food TypeMaximum Dose (kGy)Pork1Spices, vegetable seasonings and herbs30Fruit and fresh vegetables1Poultry (fresh, frozen or mechanically separated)3Red meat (fresh)4.5Red meat (frozen)7.0Shell eggs3
The FDA has granted clearance for food irradiation using ionizing radiation from three sources that have been shown to produce equivalent pathogen reductions. The approvals cover gamma rays from radioactive cobalt-60 or cesium-137, linear accelerators producing electrons at energies below 10 million electron volts, and x-rays generated from machine energies of less than 5 million electron volts.
While there are a number of large scale facilities in the United States that employ radiation technology for sterilizing of a range of products such as medical supplies, only a fraction are dedicated to food products. Common characteristics of these systems are their bulk size, scale and cost. Another common characteristic is a continuous conveyor system that supplies the food product into the irradiation chamber. Examples of such systems are disclosed in U.S. Pat. No. 4,866,281 to Bosshard, entitled Irradiation Plant; U.S. Pat. No. 5,554,856 to Bidnyy, et al., entitled Conveyor-Type Unit for Radiation Sterilization; and U.S. Pat. No. 4,481,652 to Ransohoff, entitled Irradiation Device.
Capital costs for large, production scale systems are substantial. Furthermore, the radiation sources must be shielded, often by several feet of concrete, and still provide access for food products delivered on the continuous conveyor system. To minimize per pound irradiation costs for food products, the units are often operated continuously with the typical capability to process one hundred thousand pounds of food daily. To achieve these efficiencies, the units must be located in central food processing or distribution centers that are remote from the consumer.
Machines utilizing electrons to sterilize food such as manufactured by BioSterile Technology, Inc. of Fort Wayne, Ind. are the most compact of these large production scale systems as a result of the limited penetration capability of its charged electrons. In this instance the irradiation chamber with a volume of 3.3 cubic feet is but a small component of the overall system dimensions. Nonetheless, even with a relatively small irradiation chamber, the overall size and cost of such systems remains substantial.
U.S. Pat. No. 6,212,255 to Kirk, entitled System for X-Ray Irradiation of Blood describes a smaller scale, batch irradiator for x-ray beam irradiation of blood contained within a transfusion bag. The bag is placed within a 15.5×12×4 cm canister which limits the bag to a maximum thickness of 4 cm. A first x-ray tube is positioned to irradiate the bag from a first side, and a second tube is positioned to irradiate the bag from the opposite side. An alternate embodiment includes a single x-ray tube, which is used to irradiate the first side of the bag for a preselected time period. Thereafter, the bag is rotated, and the opposite side is irradiated for an equal period of time. In either embodiment, the distance from the output port of the x-ray tube(s) to the near side of the bag is 23 cm, and the beam geometry is selected so that the beam diameter is at least 15.5 cm at this distance. Accordingly, the extent of the x-ray beam is sufficient to irradiate the entire 15.5.times.12 cm extent of the bag. One disadvantage of such a system is its bulk, as the distance between the output ports of the two x-ray tubes is some 50 cm. Of this distance, the blood to be irradiated occupies at most only 4 cm, with the remainder being air. As the intensity of the x-radiation received by the blood is inversely related to the square of the separation, the intensity of the radiation received by the bag is also relatively small. Moreover, a substantial portion of the x-rays does not impinge on the blood and therefore does not contribute to its irradiation.
U.S. Pat. No. 6,180,951 B 1 to Joehnk, et al., entitled Process for Irradiating Producing Constant/Depth Dose Profile discloses an apparatus for irradiating a target material wrapped around an annular reel. The reel is rotated about an axis perpendicular to the sweep of a beam of ionizing radiation. The objective of the arrangement is to create either a constant or a linear relationship between the depth of the target material and the received dose. Joehnk teaches that the perpendicular relationship been the axis of rotation of the reel and the direction of beam sweep is critical to the function of the invention. The electron beam source is located at a distance from the target material such that beam extent is greater than the target material extent. In addition, the target material must be wrapped around the reel and occupies only a portion of the reel's diameter. In one exemplary embodiment, a material having a thickness of 1.5 inches (3.81 cm) is disposed on a 10 inch diameter (25.4 cm) core; in another a material having a thickness of 1 inch (2.54 cm) disposed on an 8 inch (20.3 cm) diameter core. These factors likewise decrease the efficiency and increase the relative size of the apparatus.
Aspects of the present invention address these matters, and others.